Due to increasingly stringent emissions regulations, engine manufacturers are faced with a requirement to reduce all forms of emissions. Particulate emissions from diesel engines are substantially higher than petrol engines and one way to reduce the level of particulate emitted, diesel engine exhaust systems may include a particulate filter trap which catches a proportion of the particulate emitted by the engine. Over time, the filter becomes clogged with the filtered particulates and it is necessary to regenerate the filter in order to prevent excessive back pressure building up in the exhaust system which can reduce the engine's power output and eventually lead to engine failure. One known method of regenerating the particulate filter is to use the NOx generated in the engine to regenerate the particulate filter. In those systems, either the filter substrate has a catalytic coating or a separate catalyst is installed so that passing NO2 over the soot-clogged filter under certain engine conditions will cause the particulates to be broken down and the filter to be cleaned. It is also known to provide a burner system, generally fuelled by diesel fuel which, when the filter becomes clogged, heats the filter substrate to burn off the particulates. Whilst the first system is a “passive” regeneration system which relies upon a catalytic action under certain engine conditions, the latter described system is an “active” system which can regenerate the filter regardless of engine operating conditions.
Another emission that is regulated by emission controls is NOx and one method of reducing NOx production is to provide an exhaust gas recirculation system in which a proportion of the exhaust gas flowing out of the engine is returned to the air intake. This has two effects. Firstly, the exhaust gas contains a high proportion of carbon dioxide and carbon monoxide which, for the purposes of combustion in the combustion chamber are inert gases. By displacing the oxygen inducted into the combustion chamber and replacing it with carbon dioxide and carbon monoxide, the rate of NOx formation is reduced. Also, a proportion of the heat energy created by the combustion is absorbed by the carbon dioxide in the exhaust stream due to the fact that carbon dioxide has a substantial heat absorption capacity and also due to the disassociation of carbon dioxide during combustion which also absorbs energy from the combustion process. Because of that energy absorption, the combustion pressure and temperature is reduced which also reduces the production of NOx. As stated above, catalytic regeneration systems for diesel particulate filters rely on the NOx emitted from the diesel engine to regenerate the filter and to prevent the filter from becoming clogged with particulates. Accordingly, the exhaust engine is faced with a conflict between reducing the level of NOx by exhaust gas recirculation which results in less NOx being available for regeneration of the filter which, in turn, results in the filter becoming clogged or allowing more NOx to be generated by the engine in order to regenerate the filter, with the deleterious effect of addition NOx production.
It is an object of the present invention to provide an improved internal combustion engine exhaust system.